It is common knowledge that most plastics are soluble or dispersible in organic solvents. This feature is often applied in the regeneration of plastics from products consisting of different materials, in particular from waste products. With the environmental awareness recycling of plastics has attracted many attention, making use of the solvability properties of the plastics to selectively separate them from waste.
In the prior art the general idea is that the plastics are first removed from the product by mixing the product with a solvent able to dissolve or disperse the plastics. The plastics are then recovered in the form of a residue obtained by removing the solvent component from the plastics. Direct heat application to evaporate the solvent from the plastics disadvantageously yields plastics recyclates with considerable remainders of solvent, and the extensive heat load results in inferior recyclate properties.
It is known in the art that with coagulation methods often lower residual contents can be established. An example hereof is given by DE-A40 33 604, wherein a mixture of plastics and solvent is introduced in a heated coagulation bath; the following heat exchange between non-solvent and solvent results in the evaporation of the solvent and reclamation of the plastics. A drawback of the coagulation method is that the plastics then need to be freed from the coagulation medium, the method involving an additional drying step.
Moreover, these techniques often do not recover important additives in the plastics, such as the softeners in soft PVC, therewith leaving the end product with totally different—and unwanted—properties.
Alternatively, WO-A-02/31033 teaches a polystyrene reclamation process wherein waste polystyrene-type materials are dissolved in a reusable low-boiling polystyrene solvent, and wherein the solution of polystyrene-type material after concentration is devolatilised by extrusion at a temperature less than 190° C. The process involves a preheating step, in which the temperature is controlled to superheat the solution to 125-150° C., therewith enhancing evaporation. The additional evaporation step is required to concentrate the polystyrene solution to an extrudable viscosity range. Secondly, it is long known that extrusion in itself does not give satisfactory solvent evaporation, since the surface area available for evaporation in extrusion is limited. It is exactly for this reason that shearing is applied.
WO-A-02/31033 exemplifies a concentrated 33 wt % polystyrene solution, hours-lasting run times and recovery rates of 3.5-12 lbs/hr. The presence of volatiles in the recycled product may be reduced to less than 0.5%. The temperature is controlled to below 150° C. during preheating and lower than 190° C. in order to minimise the effect of heat on polymer quality. Even then, because of extended processing times, the heat loads still result in a dramatic loss of recyclate properties, especially severe molecular weight degradation. Another disadvantage of the energy-consuming extrusion process is that it is limited to a small viscosity range.
The problems existing in the art are advantageously solved by the process as disclosed in DE-A-198 29 898 A yield of reclamated plastics with a solvent content of less than 0.5 wt % is obtained even without the application of a coagulation bath. The process according to DE-A-198 29 898 comprises the above-described step of mixing waste with an organic solvent. However, instead of coagulation or extrusion, the mixture of plastics and solvent is subsequently sprayed at conditions above the temperature required to evaporate all of the solvent present. Upon spraying the overheated solvent evaporates from the optimally extended polymer solution-air interface, far more efficient that extrusion.
However, the process according to DE-A-198 29 898 is only applicable to low-viscous solutions having a plastics content of less than 40 wt %, comparable to those reported in connection to extrusion. At higher concentrations cavitations occur and a continuous throughput, especially not at high-pressure, is no longer guaranteed. Obviously, the restriction to low plastics concentrates and therewith related additional evaporation and condensation steps decrease the process efficiency, and increases the degradation of the recycled plastics, due to the prolonged and higher heat load.
In addition, with mixing techniques commercially available in the art it is only possible to separate plastics from waste in concentrations of about 10-15 wt %. It is mentioned in DE-A-198 29 898 that the separated plastics need to be concentrated to achieve 20-40 wt % throughput in the rest of the process. In WO-A-02/31033 this is achieved by the preheating step. Disadvantageously, such inefficient mixing results in high solvent and energy demands, also because of the limited energy capacity of a low plastics concentration.
Finally, the reclamation processes in the art, including the process according to DE-A-198 29 898, are disadvantageously affected by water accumulation in the solvent. Hence, the products need to be dried prior to submittal to the recycling process and the water-contaminated solvent is subjected to an additional cleansing step before it can be reused.
It is therefore an aim of the invention to provide a process for recycling plastics from waste material with an increased yield of plastics, wherein higher concentrations of plastics can be handled and wherein water contamination of the separated plastics and the applied organic solvents can be maintained at low levels.